Many frequency bands are used in modern communications systems. Mobile devices, for example, may use five different cellular radio bands plus WLAN, Bluetooth® and mobile TV bands. Each frequency band requires a separate antenna design and so an antenna company has to have many different products on its books and carry a variety of different stock.
It is known, for example from WO 2005/022688, to provide a modular antenna apparatus in which an antenna can be built up from a selection of modules having differing resonance frequencies, the modules being connected in series along a connection conductor. Open terminals of the antenna modules are separate. The antenna structures formed from the modules are relatively simple, and are provided with only a single effective feed port.
In addition, it is a known technique to form an antenna from a branched conductor system in order to increase the bandwidth of a single radiating element or to provide an antenna which operates in more than one frequency band. An example is illustrated in Figure 28-5b in Antenna Engineering Handbook (4th Edition, Editor J Volakis, published by McGraw-Hill Book Company, New York, 2007); this design was originated in the 1940s and has been sold commercially and constructed by radio amateurs for many years for use in the HF radio band (3-30 MHz).
JP 2002-335114 discloses a chip antenna designed so that its resonance frequency can be changed adaptively. The chip antenna comprises a meandered conductor embedded in a chip, and three terminals connected to different points on the conductor and all projecting from one edge of the chip. In this way, depending on the terminal selected as the feed, three different lengths of conductor and hence three difference resonance frequencies are immediately available. It is possible to trim the non-feed terminals so as to provide additional tuning.